Linearly polarized fiber laser usinga point-by-point Bragg grating

in a single-polarization photonic bandgap fiberRyuichiro Goto,1,* Robert J. Williams,2,3 Nemanja Jovanovic,3 Graham D. Marshall,2,3

Michael J. Withford,2,3 and Stuart D. Jackson1

1Institute of Photonics and Optical Science, School of Physics, University of Sydney, New South Wales 2006, Australia2Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), Macquarie University, New South Wales 2109, Australia3MQ Photonics Research Centre, Department of Physics and Astronomy, Macquarie University, New South Wales 2109, Australia

*Corresponding author: ryugoto@physics.usyd.edu.au

Received March 3, 2011; revised April 7, 2011; accepted April 16, 2011;posted April 20, 2011 (Doc. ID 143603); published May 13, 2011

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-pointBragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating waswritten within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. Thisall-fiber laser produced 7:2W, linearly polarized output with 25pm FWHM and 19:6 dB polarization extinctionratio. © 2011 Optical Society of AmericaOCIS codes: 060.2310, 060.2400, 060.3735, 060.3510, 230.5440, 320.2250.

Narrow-linewidth, linearly polarized fiber lasers arecritical components for many applications, such as non-linear frequency conversion [1] and beam combining toproduce higher power [2]. Single-polarization (SP) fibersguide only one polarization with low loss within a certainwavelength band (SP band), and are commonly used asfiber-based polarizers in linearly polarized fiber lasers[3,4]. The incorporation of fiber Bragg gratings (FBGs)is highly desirable for enabling narrow-linewidth outputfrom such linearly polarized fiber lasers, because the useof an SP fiber and FBGs enables an all-fiber cavity con-figuration, which improves the efficiency and robustnessby eliminating free-space bulk optics. We have previouslyreported a highly birefringent all-solid photonic bandgap(PBG) fiber operating as an SP fiber [5]. All-solid PBGfibers have high-index regions surrounding the core todefine discrete transmission bands via the PBG effect[6]. These high-index regions make FBG inscription usingUV illumination techniques highly problematic: the high-index regions are typically more photosensitive than thecore, thus, the PBG transmission bands become dis-torted. For example, in [7] the short-wavelength edgeof one of the transmission bands was shifted by 80 nmin the exposed length of fiber, thus drastically narrowingthe net transmission band of the whole fiber. This hasbeen overcome by using phosphorus-doped high-indexregions; however, it resulted in high propagation lossin the fiber [8]. We overcome these issues by inscribingFBGs with a femtosecond laser using the point-by-point(PbP) technique [9]. This technique does not requirephotosensitization of the core, and the refractive indexmodifications may be highly localized within the core,enabling FBG inscription without unwanted effects onthe PBG transmission characteristics of the fiber.In this Letter, we report a multiwatt, narrow-linewidth,

linearly polarized all-fiber laser using the aforementionedSP, all-solid PBG fiber, incorporating a narrow-linewidthPbP FBG. Polarization dependent effects in PbP FBGshave proven to be effective for linearly polarized lasingup to several tens of milliwatts [10]; however, the small

polarization extinction ratio limits power scaling to thewatt-level. An FBG inscribed in the PBG fiber at theSP band enables highly polarized reflection suitablefor power scaling. The PBG fiber including the gratingwas incorporated in an Nd-doped fiber laser as an inte-grated polarization- and wavelength-selective reflector,and this all-fiber laser produced 7:2W, narrow-linewidth(25 pm FWHM), linearly polarized laser output with 42%slope efficiency.

Figure 1 shows the SP all-solid PBG fiber used for PbPgrating inscription. The structure of the fiber is similar toour previous report [5], except that the current fiber has asmaller diameter (125 μm). The fiber is based on a hybridstructure [11] where the core modes are guided both bythe PBG effect and total internal reflection (TIR). Thegermanium-doped high-index regions enable guidancevia the PBG effect, as well as inducing stress birefrin-gence to the core. The boron-doped stress-applying partsfurther enhance the stress birefringence of the core. Thefluorine-doped low-index uniform cladding enables gui-dance by TIR. The calculated mode field diameter is7:3 μm at 1060 nm in the direction of the low-index clad-ding. Figure 2(a) displays the transmission spectrum ofthe fiber for two orthogonal polarizations. Large birefrin-gence in our fiber shifts transmission bands for the twopolarizations, enabling a 25nm SP band at the short-wavelength edge of the second PBG. Figure 2(b) showsthat the SP band shifts to longer wavelengths by coiling

Fig. 1. Cross section of the SP all-solid PBG fiber.

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the fiber, enabling tuning of the SP band. Birefringenceand effects of bend of the fiber are described in [5].The PbP technique used for FBG inscription is de-

scribed in [12]. Laser pulses from a regeneratively ampli-fied Ti:sapphire femtosecond laser (<110 fs duration at800 nm, 1 kHz repetition rate) were focussed into thecore of the PBG fiber using a 0.8 NA, oil immersion ob-jective lens. The fiber was stripped of its polymer jacketand threaded through a glass ferrule, which was mountedin front of the objective lens. The fiber was rotated suchthat the laser pulses were focused to the core through thelow-index cladding. A 20mm uniform FBG with period of1:102 μm was inscribed using 170 nJ laser pulses. ThisFBG had a third-order Bragg resonance at 1064 nm.The micrograph shown in Fig. 3 clearly displays themicrovoids in the core, with the high-index regions ap-pearing above and below the core.Figure 4(a) shows the transmission spectrum of the

PbP FBG, measured using polarized light (from a fiberamplified spontaneous emission source) with the polar-ization axis aligned to the slow axis of the PBG fiber. Thetransmission of the fiber was measured immediatelyprior to grating inscription in order to obtain a normal-ized grating transmission spectrum. The transmission ex-tinction at the Bragg wavelength is 9dB, and nonresonantscattering loss measured outside the Bragg reflectionband was ∼1:5 dB. We estimate the reflectivity at theBragg wavelength to be 70% (with 12% transmissionand 18% nonresonant loss). The nonresonant loss at

the Bragg wavelength is less than that outside the Braggreflection band because the reflected light does not fullypenetrate the grating. Figure 4(b) shows the reflectionspectrum of the FBG, measured using unpolarized light.The spectrum without coiling of the fiber shows twoBragg resonances, corresponding to the fast and slowaxes of the fiber. Upon coiling the fiber, the reflectionat the shorter wavelength was eliminated as a result ofthe SP band shifting to longer wavelengths (see Fig. 2);hence, the effective reflection from the grating becamepolarized. The FWHM of the Bragg reflection was lessthan 0:2 nm. Narrower grating linewidths may be ob-tained using lower inscription pulse energy and a longergrating length [12].

The SP-PBG fiber (including the FBG) was incorpo-rated in a fiber laser cavity as shown in Fig. 5(a). A10m Nd-doped polarization-maintaining double-clad fi-ber (Nufern, PM-NDF-5/125) was pumped by a 808 nmmultimode diode laser through a dichroic mirror(T>95% at 808 nm and R > 99% at 1064 nm), and morethan 90% of the launched pump light was absorbed in thefiber. We used Nd to simplify our experiment using afour-level transition; however, in principle a number ofrare-earth dopants can be used. The Fresnel reflectionfrom the cleaved end-facet of the Nd-doped fiber servedas the output coupler. The PBG fiber was spliced tothe Nd-doped PM-DC fiber with the polarization axesaligned. Index matching gel was applied to the strippedPBG fiber to eliminate unwanted feedback, and anypotential thermal effects due to the scattered light fromthe FBG. The PBG fiber between the FBG and splicepoint was coiled to tune the SP band onto the Braggwavelength. The laser output was extracted via thedichroic mirror. The laser produced 7:2W output at17:5W pump power with 42% slope efficiency, as shownin Fig. 5(b). The polarization extinction ratio measured at7:2Wwas 19:6 dB. Figure 6 displays the laser spectrum at

Fig. 2. (Color online) Polarization dependent transmissionspectra of a 2m PBG fiber when (a) the fiber was not coiledand (b) a 65mm-diameter two-turn coil was applied to the fiber.The small peak in the spectrum at 1064 nm is due to the pump ofthe supercontinuum source used for this measurement.

Fig. 3. (Color online) Transmission differential interferencemicrograph of the PbP grating.

Fig. 4. (Color online) (a) Normalized transmission spectrum(slow axis) and (b) reflection spectrum of the PbP grating(unpolarized, and slow axis).

Fig. 5. (a) Experimental setup of the linearly polarized fiberlaser and (b) slope efficiency of the laser.

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7:2W; a high-resolution measurement in the inset showsthe 25 pm FWHM linewidth of the laser. In order to quan-tify loss elements in the cavity (the splice, the coil in thePBG fiber, and the <100% reflectivity of the FBG), wecharacterized the laser with incremental changes tothe cavity: (1) by uncoiling the PBG fiber, the laser pro-duced unpolarized, narrow-linewidth output with 44%slope efficiency; (2) by cutting the PBG fiber to removethe FBG and butt-coupling a high-reflector dielectricmirror to the PBG fiber, the laser produced unpolarized,broadband output with 47% slope efficiency; and (3) byremoving the PBG fiber and butt-coupling the high-reflector mirror to the Nd-doped fiber, the laser producedunpolarized, broadband output with 53% slope efficiency.Therefore, we attribute the following penalties in slopeefficiency to the following loss elements in the cavity:the splice loss (6%), the <100% reflectivity of the FBG(3%), and coiling loss of the PBG fiber for the slow axis(2%). The 6% penalty due to the splice loss could be elimi-nated by incorporating a doped core into the SP-PBGfiber. At 20W pump power we observed two weaksecondary peaks near 1068 nm, 27 dB below the lasingpeak; however, the slope efficiency and the polarizationand linewidth of the main peak were maintained. Weattribute these secondary peaks to increased long-wavelength gain in the Nd-doped fiber, which weobserved with increasing pump power. A PbP FBGwritten in a Yb-doped fiber has enabled narrow-linewidthoutput up to 100W [13] without this issue; therefore, weexpect our system will be similarly power scalable usinga Yb-doped fiber.In conclusion, we have demonstrated a narrow-

linewidth, linearly polarized fiber laser that used a PbPBragg grating written into an SP all-solid PBG fiber.Our fiber laser demonstrates a compact and robust plat-

form by integrating the polarizer and reflector into onefiber. The use of an all-solid PBG fiber potentially enablesan all-fiber linearly polarized fiber laser that includesother features of an all-solid PBG fiber, such as alarge-mode area [14] and gain suppression [14,15].

Fujikura Ltd. is acknowledged for providing the PBGfiber. The authors thank Alex Butler and David Spence atthe MQ Photonics Research Center for the loan of essen-tial equipment used for this work. R. Goto acknowledgesthe Department of Education, Employment and Work-place Relations, Australia, for financial support. Thiswork was produced with the assistance of the AustralianResearch Council under the ARC Discovery Projects,Centers of Excellence and LIEF programs.

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Fig. 6. Output spectrum of the laser measured at 7:2W outputpower (log scale). The inset shows the linewidth of the laser(linear scale).

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